1. Field of the Invention
The present invention relates generally to trap devices that reduce the volume of at least one undesired constituent present in gases passing therethrough and generally through an associated vacuum system.
2. State of the Art
The generic names for the devices that remove constituents from a gas stream are a trap, a cold trap, or a byproduct trap. Trap devices may be typically used in combination with vapor phase reaction processes, such as Chemical Vapor Deposition (CVD), including without limitation the application of CVD to so-called Atomic Layer Deposition (ALD), to remove undesirable constituents from the gas that is used to perform the vapor deposition as it is removed from the deposition chamber. More particularly, process gas may be removed from the treatment chamber, as part of a semiconductor manufacturing environment, by way of a vacuum pump or other vacuum source as known in the art. Thus, gases may be typically captured prior to reaching the vacuum pump by a trap device connected between the chamber outlet and the vacuum pump used to pump residue gases from the vacuum chamber. A primary reason for removing undesirable constituents from residual process gas prior to reaching the vacuum pump is to protect the vacuum pump from excessive wear, undesirable depositions on components thereof, chemical reactions with pump components, or other undesirable effects on the vacuum pump that may be caused by gases passing therethrough.
Of course, trap devices may be designed for treating specific exhaust gases that are derived from particular processes since different processes will use different processing gases and may exhibit different conditions. For instance, in semiconductor device fabrication, titanium films may be deposited by the general reaction between titanium tetrachloride and silane. Unfortunately, the titanium tetrachloride that does not react with silane to form titanium may be deleterious to the vacuum pump. Therefore, a trap device may commonly be used in semiconductor manufacturing CVD systems for removing titanium tetrachloride that exits the CVD chamber.
It is, therefore, to be expected that a variety of trap devices are available, whereby each type is aimed at a particular processing environment. These different types of trap devices may be configured for different constituents that may be contained in the exhaust gases, in different concentrations and for different responses of the exhaust gases to sudden cooling. Also, a number of chemical substances may be removed by passing the gases through a filter that removes these constituents without the benefit of a rapid change in temperature of the gases. However, removal of a target constituent is, in most cases, not so complete that the gases that have passed through a filter may not need further processing.
The principle that is most frequently used in the operation of trap devices is one of cooling a gas that is to be removed from a residual process stream, thus causing the gas to condense and accumulate inside the trap device. Of course, the trap device eventually fills with condensate residue which must then be removed by cleaning. Often, a trap device may be equipped with a series of tubes or baffles that are cooled and that intercept and contact the gases that flow through the trap device, thereby causing the gases to condense. One purpose of the tubes or baffles within the trap device is to cause the gas that passes through the device to be exposed to a particular reduced temperature over as long a period of time as possible. In so doing, the probability of collisions between gas molecules and the baffle or tube surfaces may be increased, leading to improved trapping efficiency for gases or other reaction by-products. However, by increasing the length of the path that the gases travel as they move through the trap device, it may require more frequent cleaning because the sizes of the apertures within the trap device that the gas passes through may be reduced prematurely. Stated another way, deposits may form unevenly within a trap device and constrict the passage of gases therethrough because of uneven temperature distributions within the trap device, or because there is more constituent material in the gas stream to be removed as the gas enters the trap than when the gas exits the trap, resulting in increased deposits near the inlet.
Thus, the deposits within a trap device may be distributed unevenly and passages through these devices may become plugged or obstructed by uneven distribution of deposits therein. Such uneven deposition of deposits within the trap device has undesirable effects. First, a substantial amount of capacity of the trap device may not be utilized because the uneven distribution of deposits may cause the trap device to become unusable before being completely full. Moreover, the trap device must be cleaned more often, which may particularly impact a manufacturing environment in lost manufacturing time. By way of example only, conventional trap devices may require cleaning after as few as 500 semiconductor wafers are processed through an associated process chamber such as a CVD chamber, which deficiency may provide an operational time for the system including the process chamber of as little as a day and a half before cleaning of the trap device is required.
FIGS. 1A and 1B show an exemplary, conventional trap device 10 generally defined by a cooling assembly 11 disposed within a cylindrical housing 22, with FIG. 1B depicting trap device 10 in a partially disassembled state, such as for cleaning. Top plate 28 may be removable from the cylindrical housing 22 and may be temporarily affixed thereto by bolts or a compression fitting and sealed thereto by an o-ring seal or as otherwise known in the art. Cooling assembly 11 may comprise tubing used to form cooling inlet 16 and cooling outlet 18 and cooling coils 20 therebetween. Cooled fluid or gas may be passed through the cooling inlet 16, cooling coils 20, and the cooling outlet 18 to remove heat from the conventional trap device 10. Chilled water or any other suitable fluid or gas may be used, as known in the art. As heat is removed from gases passing through conventional trap device 10, condensation and/or freezing of the gases may occur.
During operation, gases pass through the vacuum inlet 12 and are directed via inlet deflection plate 34 toward the outer diameter of the cylindrical housing 22. Gases then travel along the outer annulus 36 formed between outer deflection tube 24 that extend vertically downward from the inlet deflection plate 34 and the wall 42 of the cylindrical housing 22. Further, baffles 32 extending between the wall 42 of the cylindrical housing 22 and outer deflection tube 24 may cause the flow path of the gases passing thereby to be deflected radially as the gases move downwardly along outer annulus 36. Upon reaching the lowest extent of the outer deflection tube 24, the gases move into annulus 38 formed between outer deflection tube 24 and the vertical structure comprising the coils 20 and coil separation elements 30 and sealing element 31. Separation elements 30 may be installed between coils 20 for structural support, or, alternatively, the separation elements may be omitted by positioning coils 20 proximate to one another and then affixing the coils 20 to one another via brazing or as otherwise known in the art. Sealing element 31 may be configured to engage and seal against the bottom inner surface 44 of the cylindrical housing 22 as the top plate 28 and cooling assembly 11 are installed within the cylindrical housing 22. As gases travel through annulus 38 they may be deflected by way of baffles 32 that extend therein. Thus, gases may condense on the outer deflection tube 24, on the coils 20, and on the baffles 32 as the gases travel through and interact with the cooled surfaces thereof. In addition, as may be seen in FIG. 1A, the gases continue to the upper end of the coils 20, and then may move radially inwardly into annulus 40, also traveling along and around the baffles 32 that extend between the inner deflection tube 26 and the coils 20. Inner deflection tube 26 may be affixed to the cylindrical housing 22 at the bottom inner surface 44 and may be configured to engage and seal against the surface of outer deflection plate 34. Alternatively, the inner deflection tube 26 may be affixed to the inlet deflection tube 24 and removed therewith for cleaning, as depicted in FIG. 1B. Aperture 45 formed in inner deflection tube 26 allows gases to move through the trap device 10 and eventually exit the trap device 10 through vacuum outlet 14.
As is illustrated in FIG. 1C, deposits 13 may form within the trap device 10, on the baffles 32, the wall 42 of the cylindrical housing 22, the coils 20, the separation elements 30, and/or the sealing element 31, as well as on any surface within the trap device which interacts with the gases passing therethrough. Furthermore, deposits 13 may form unevenly, as shown in FIG. 1C. One reason for uneven distribution is that the cooling medium passing through the coils 20 may enter at a first temperature at the top of the trap device 10 and, as it passes through the coils 20, may be warmed as gases condense within the trap device 10. Therefore, the temperature of the coils 20 and baffles 32 attached thereto may be cooler near the inlet (top) of the trap device 10 than near the outlet (bottom) of the trap device 10. Thus, deposits 13 are shown as being relatively thick near the vacuum inlet 12 and top of outer annulus 36, as well as near the top of coils 20 between annulus 38 and annulus 40. As may be seen, the formation of deposits 13 may prevent the trap device 10 from functioning if the deposits reduce the ability of the vacuum inlet 12 to communicate with the vacuum outlet 14. Eventually, deposits may prevent communication between the vacuum inlet 12 and the vacuum outlet 14. As is further illustrated by FIG. 1C, uneven distribution of deposits 13 within trap device 10 may cause cleaning to become necessary after a relatively small volume of deposits forms within the trap device 10. Moreover, it may be seen that if deposits 13 were more evenly distributed within the trap device 10, the trap device 10 may require cleaning less frequently and may continue to operate to contain a greater amount of deposits 13 accordingly.
Of course, many different embodiments of conventional trap devices are possible, and FIGS. 1A–1C merely illustrate one such conventional design. Further, trap devices may be cooled by other means such as liquid nitrogen, dry ice, cooled gases, or thermoelectric devices as known in the art. In such configurations, normally, a vacuum chamber and a cooling chamber share a common wall, so that the cooling medium within the cooling chamber removes heat from the vacuum chamber, thus condensing and freezing the gases passing through the vacuum chamber.
As may be seen from FIGS. 1A–1C, the path of gases traveling through the conventional trap device 10 is intended to lengthen the path that the gases must traverse so that interaction time between the gases and the cooled surfaces within the conventional trap device 10 is increased and the gases may be condensed and thereby trapped more efficiently. However, as may also be seen by FIGS. 1A–1C, lengthening the path that the gases must follow decreases the relative cross-sectional area of the path that the gases must pass through for a given volume within a trap device. Thus, if the gases condense unevenly, the deposits 13 may accumulate and prevent gases from passing through the conventional trap device 10, thus necessitating removal of the cooling assembly 11 for cleaning. Uneven deposits 13 may be caused by any number of conditions such as the temperature distribution of the cooling assembly 11 and the cylindrical housing 22, the characteristics of the flow (such as turbulence) of the gases, as well as the distance along the path in relation to the vacuum inlet 12.
U.S. Pat. No. 6,241,793 B1 to Lee et al. discloses a curvilinear housing and a curvilinear cooling tube contained therein to reduce the frequency of cleaning of the cold trap. The cooling plate may also include a plurality of fins disposed thereon, generally facing the inlet of the housing, and spaced equidistantly from one another.
U.S. Pat. No. 6,206,971 B1 to Umotoy et al. discloses a temperature-controlled exhaust assembly with cold trap capability and multizone closed-loop temperature control. More specifically, as to the trap apparatus, Umotoy utilizes an external heater around the inlet of a cold trap to prevent buildup therein.
U.S. Pat. No. 6,528,420 B1 to Tong et al. discloses a double-acting cold trap including a deflecting plate that directs exhaust gases first over condensing fins and then over plates that are oriented perpendicular to the flow of the gases. The geometry and arrangement of the fins and plates are directed toward increasing the time between cleaning cycles by way of increasing the available area for condensate to be deposited.